First of all, description will now be made in relation to a sensitivity and a contribution rate that are to appear in the following.
A sensitivity of each of the parts constituting a structure (an assembly) is determined by the formation of the structure, irrespective of the size tolerances of each part, and is a degree of influence of the part on the size (the design specification value) of a predetermined measurement object portion (e.g., a portion which demands a predetermined quality when the object is to be assembled) of the structure.
More specifically, a sensitivity is a variation rate in the size of a measurement object portion relative to a variation in size of each part.
Accordingly, a part with a higher sensitivity is more important to the quality of the design specification value (in other words, the quality of the structure).
As shown in FIG. 13(a), in structure C formed by parts A and B, the variation in the size of measurement object portion (gap) G results in elongation of 1 mm if the size of part A is elongated by 1 mm in the direction of arrow a. Accordingly, the sensitivity of part A is “1”.
Further, as shown in FIG. 13(c), since part B is inclined at 45 degrees to measurement object portion G, elongation of the size of part B by 1 mm in the direction of arrow B results in variation g′ in the size of the measurement object portion calculated to be “0.707” by following equation (1)g′=cos 45°×1  (1)
The sensitivity of part B is consequently “0.707”.
Next, a contribution rate is detailed. A contribution rate of each of the parts is a ratio of a size tolerance of the part to the total sum (the deviation) of size tolerances of the parts that form a measurement object portion (a design specification value). For this reason, a part with a higher contribution rate has a size tolerance larger than those of the remaining parts.
Specifically, if structure D consists of parts D1 and D2 as shown in FIGS. 14(a)-14(c), the contribution rates of parts D1 and D2 are as follows.
As shown in FIG. 14(a), if part D1 has size deviation of “1.0” because the size tolerance of part D1 is “±0.5” and part D2 has size deviation of “2.0” because the size tolerance of part D2 is “±1.0”, the size deviation (i.e., the total sum of the size tolerances of parts D1 and D2) at measurement object portion G′ is “3.0” as shown in FIG. 14(c) because sensitivities of part D1 and D2 are both “1”.
As a result, the contribution of part D1 is “33%” and that of part D2 is “66%”.
Conventionally, in designing a structure formed by a number of parts, the designer calculates size tolerances (design specification value) of a portion (a measurement object portion) that requires a predetermined quality in assembly of the structure by manual calculation of the root sum square based on size tolerances of the parts or by estimation based on experience.
However, both methods fail to consider the three-dimensional shape, and if a structure has a complex shape or consists of parts assembled densely, the calculation and inputting (of the sizes and the like) for the calculation tend to be complex and to cause errors.
If a calculation error or input error has occurred, assembly of a structure cannot satisfy the design specification value and results in, at worst, return to the design stage because the structure cannot be assembled.
To avoid this, there recently have been provided tolerance analysis systems utilizing, for example, three-dimensional CAD data (see below Patent References 1-3, for example).
In a conventional tolerance analysis system, tolerance analysis (that is, decision of size tolerances) is performed through the procedure shown in flow diagram FIG. 15 (steps S100-S108).
Specifically, in a conventional tolerance analysis system, design data (shape data, here three-dimensional CAD shape data) of a structure formed by a number of parts is obtained from a design unit using a CAD system or the like (step S100).
With reference to the obtained design data, the designer (the operator of the tolerance analysis system) sets the design specification value of a measurement object portion of the structure (step S101) and inputs size tolerances of each of the parts (step S102) via an interface such as a mouse and/or keyboard.
Further, the designer sets production process data (e.g., a production method and/or the accuracy) for each part according to the material and the production process of the structure (assembly process of part) (step S103).
The arithmetic unit (e.g., the CPU: Central Processing Unit) of the tolerance analysis system starts the tolerance analysis (step S104) and as a result automatically calculates the quality (a value; deviation) to the design specification value, and the sensitivity and the contribution rate of each part (step S105).
In succession, the designer judges whether or not the quality calculated as the result of the tolerance analysis reaches a desired quality (step S106).
Here, if the designer judges that the calculated quality reaches the desired quality (Yes route in step S106), the arithmetic unit outputs the size tolerances, which have been input by the designer in step S102, to complete the tolerance analysis (step S107). Finally, the procedure terminates.
On the other hand, if the designer judges that the calculated quality does not reach the desired quality (No route in step S106), the designer re-examines the size tolerances of each part (step S108) by considering the balance of the entire structure with reference to the sensitivities and the contribution rates of the parts calculated by the arithmetic unit in step S105. On the basis of the result of reexamination, the renewed size tolerances of the parts are input again (step S102).
Namely, until the designer judges that the calculated quality reaches the desired quality (Yes route in step S106), the processes of above steps S108 and S102-S105 are repeated.
A conventional tolerance analysis system calculates size tolerances of the parts which satisfy the desired quality in the manner above described.    Patent Reference 1: Japanese Patent No. 2820170    Patent Reference 2: Japanese Patent Application Laid-Open No. 2002-82995    Patent Reference 3: Japanese Patent Application Laid-Open No. 2003-6241